Priming allows seed germination processes to begin by partially hydrating seed but at sufficiently low water potentials such that radicle emergence does not occur; seed are typically dried-back to their dry state after the procedure. Specialized priming techniques may include hydropriming, halopriming, osmopriming, biopriming, and solid matrix priming (Ashraf, M. and M. R. Foolad. 2005. In: Advances in Agronomy. D. L. Sparks ed. vol. 88. pp 223-251. Elsevier Academic Press. San Diego, Calif.). Seed physiological improvement following priming may be due to advanced embryo growth, breakdown of endosperm tissue, and synthesis of beneficial macromolecules (reviews by Ashraf, M. and M. R. Foolad. 2005. In: Advances in Agronomy. D. L. Sparks ed. vol. 88. pp 223-251. Elsevier Academic Press. San Diego, Calif.; Khan, A. A. 1992. Hort. Rev. 13:131-181). Osmotic priming subjects seeds to low osmotic potential at a specific temperature and duration and has been shown to increase germination rate, emergence, and/or stand size and uniformity of carrot (Brocklehurst, P. A. and J. Dearman. 1983. Ann. Appl. Biol. 102:577-584; Haigh, A. M. and E. W. R. Barlow. 1987. J Amer. Soc. Hort. Sci. 112:202-208; Pill, W. G. and T. A. Evans. 1991. J Hort. Sci. 66:67-74; Pill, W. G. and W. E. Finch-Savage. 1988. Ann. Appl. Biol. 113:383-389; Szafirowska, A., A. A. Khan, and N. H. Peck. 1981. Agron. J. 73:845-848). Osmotic priming employs chemicals (polyethylene glycol or salt solutions).
Solid matrix priming involves suspending seeds in a defined solid (matrix) of known matric potential to control seed water intake. The solid can be organic, inorganic or both. The seed and matrix compete for available water in a controlled aeration and defined temperature environment for a prescribed priming period. (S. M. Hock, S. Z. Knezevic, C. L. Petersen, J. Eastin and A. R. Martin. 2006. Weed Technology 20(2):530-534) Commercial priming carriers for solid matrix priming include Zonolite® (vermiculite, Grace Construction Products), vermiculite #5 and Celite® Micro-Cel E (calcium silicate, CaSiO3). Peat moss, sand, bituminous soft coal, Leonardite shale, and calcined clay are also solid carriers that can potentially be used for priming. Current solid carriers for solid matrix priming must be mined from specific sources and refined prior to use. Other organic materials used in priming such as peat moss have high carbon contents, must be mined or harvested from environmentally sensitive areas and must be shipped significant distances and be processed before use. Priming subjects seeds to sufficiently low matric potential such that radicle emergence is prevented but physiological processes are initiated.
There remains a need for effective, low-cost methods for seed priming that do not employ mined materials. At present, there is no report of a compost product that generates adequate water potential and possesses sufficient water retention for commercial seed priming for significant durations beyond twenty-four hours.